Our long range goals are to define the roles and to understand the relationship between the roles of cyclic AMP and cyclic GMP in biological systems. We will use a series of xanthines, some of which are potent inhibitors of cyclic nucleotide phosphodiesterase and are relatively selective for one of the forms of the enzyme isolated from pig coronary arteries, as tools to help define the roles of the cyclic nucleotides if it can be established that the pharmacological effects of these xanthines arise from their ability to inhibit phosphodiesterase. The specific goals of this study are to quantitate the effects of these xanthines to (1) alter the contraction of smooth, cardiac and skeletal muscle, (2) inhibit the phosphodiesterases, and (3) cause changes in cyclic AMP and cyclic GMP levels in these tissues.